A so-called wafer-level package (WLP) type acoustic wave device directed at achieving compactness is known. This WLP type acoustic wave device has a piezoelectric substrate, excitation electrodes provided on the piezoelectric substrate, and a cover sealing the excitation electrodes (refer to, for example, Patent Reference 1).
The cover is formed by an annular frame member surrounding the excitation electrodes disposed on the upper surface of the piezoelectric substrate and a lid member disposed on the upper surface of the frame member so as to cover the aperture of the frame member. The lid member covering the aperture of the frame member in this manner establishes an oscillation space above the excitation electrodes, which is a region surrounded by the inner wall of the frame member and the lower surface of the lid member.
Although the frame member and lid member making up the cover are often formed from the same material, there are cases in which a small gap is formed between the frame member and the lid member in the case, for example, of the frame member and the lid member being formed by separate processes.
If a gap is formed between the frame member and the lid member, a linking of the outside of the cover and the oscillation space via the gap changes the atmosphere within the oscillation space. Water from the outside air entering the oscillation space via the gap can cause corrosion of the excitation electrodes. If the excitation electrodes corrode, the electrical characteristics of the acoustic wave device deteriorate.
Given the above, it is desirable to provide an acoustic wave device that, even if the cover is formed by a frame member and a lid member, can maintain airtightness of the oscillation space over a long period of time.